EP0104740B1 - Broad band surface acoustic wave edge deposited transducer - Google Patents

Broad band surface acoustic wave edge deposited transducer Download PDF

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Publication number
EP0104740B1
EP0104740B1 EP83304744A EP83304744A EP0104740B1 EP 0104740 B1 EP0104740 B1 EP 0104740B1 EP 83304744 A EP83304744 A EP 83304744A EP 83304744 A EP83304744 A EP 83304744A EP 0104740 B1 EP0104740 B1 EP 0104740B1
Authority
EP
European Patent Office
Prior art keywords
layer
substrate
transducer
acoustic wave
surface acoustic
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
EP83304744A
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German (de)
English (en)
French (fr)
Other versions
EP0104740A3 (en
EP0104740A2 (en
Inventor
Robert S. Wagers
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Texas Instruments Inc
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Texas Instruments Inc
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Filing date
Publication date
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Publication of EP0104740A2 publication Critical patent/EP0104740A2/en
Publication of EP0104740A3 publication Critical patent/EP0104740A3/en
Application granted granted Critical
Publication of EP0104740B1 publication Critical patent/EP0104740B1/en
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Classifications

    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H9/00—Networks comprising electromechanical or electro-acoustic elements; Electromechanical resonators
    • H03H9/02—Details
    • H03H9/125—Driving means, e.g. electrodes, coils
    • H03H9/145—Driving means, e.g. electrodes, coils for networks using surface acoustic waves
    • H03H9/14502—Surface acoustic wave [SAW] transducers for a particular purpose

Definitions

  • This invention relates to surface acoustic wave devices and more particularly to a device for producing surface acoustic waves at high frequencies and broad bandwidth on non-piezoelectric or weakly piezoelectric substrates.
  • EBT edge-bonded transducer
  • An electrode is fabricated on the outer edge of the EBT and excited with respect to the conductive bond, generating shear waves that convert to surface waves.
  • This technique has been described in the article by D. E. Oates and R. A. Becker, "LiNb0 3 Surface Acoustic Wave Edge Bonded Transducer on S. T. Quartz and (001) Cut GaAs", 1980 Ultrasonics Symp. Proc. (1980), pp. 367-370, with an LiNb0 3 X-cut shear-wave transducer indium-bonded to the GaAs substrate. Shear waves generated in the edge-bonded transducer are converted to surface waves in the GaAs substrate. Low insertion loss and fractional bandwidths of 91 percent at 100 MHz were reported for the GaAs EBT.
  • the EBT can potentially lead to wideband low-loss transduction in GaAs, its operation is limited to center frequencies below 200 MHz.
  • Technical difficulties with lapping, bonding, and polishing the LiNb0 3 transducer prevent higher frequency operation of the EBT.
  • the thickness of the half wavelength LiNb0 3 transducer would be approximately 6.8 um. This is less than half the thickness of the transducers used in the before-mentioned article and would present a serious materials fabrication problem.
  • the required thickness is reduced to approximately 1.7 ⁇ m, a thickness clearly unobtainable.
  • Another object of the present invention is to generate surface-acoustic wave excitation with high frequency and with a large bandwidth.
  • Another object of the present invention is to provide a transducer which will generate surface acoustic waves in a semiconductor substrate at high frequencies and large bandwidth.
  • Another object of the present invention is to provide a device which can be totally fabricated on a monolithic semiconductor material.
  • Another object of the present invention is to fabricate an edge deposited transducer which provides low spurious signal generation in the form of reflections, bulk waves and dispersion.
  • a surface acoustic wave device comprising: a) a substrate for transmitting surface waves, said substrate having a top surface and a side surface forming a sharp, smooth edge at their intersection; and b) a transducer portion acoustically coupled to the side surface of said substrate and comprising: i) a layer of transducer material having first and second opposing surfaces, ii) an inner metallic electrode for connection to a signal source, formed on the first surface of the transducer layer, and iii) an outer metallic electrode for connection to the signal source formed on the second surface of the transducer layer, said transducer portion operatively affixed to said substrate characterised in that the outer electrode is positioned a predetermined distance below the top surface of said substrate so that when an excitation signal is applied to the inner and outer electrodes to generate bulk longitudinal waves in said transducer layer surface waves are produced in said substrate, said predetermined distance being at least as great as the distance from top surface of said substrate to a point along
  • Transducer 10 is positioned on a substrate 12; this substrate 12 may be, for example, gallium arsenide (GaAs), silicon (Si), indium phosphide (InP) or quartz.
  • GaAs gallium arsenide
  • Si silicon
  • InP indium phosphide
  • quartz quartz
  • This transducer solves a particular problem by providing a method for carrying out acoustic wave excitation on semiconducting materials, such as silicon and gallium arsenide. Silicon is non-piezoelectric and does not provide an inherent means for surface wave excitation while gallium arsenide is weakly piezoelectric and inherently permits only narrow band surface wave excitation with interdigital transducers.
  • gallium arsenide is preferred because of its unique combination of acoustic, semiconducting and optical properties.
  • lossless surface waves may be generated in substrate 12 in the (110) direction indicated by arrow 14.
  • Substrate 12 should be prepared such that its side portion 16 and top portion 18 form a smooth edge 20 without irregularities and chips that intersect top surface 18 where the surface acoustic wave 22 will propagate.
  • Gallium arsenide and silicon lend themselves to this fabrication step through cleaving.
  • a passivation layer 24 (see Figure 2) is applied to substrate 12, if necessary, to prevent the intermixing or diffusion of substrate materials and transducer materials.
  • This passivation layer may be comprised of a plurality of separate layers. If quartz, by way of example, is used as the substrate 12, a passivation layer is not required since the inner electrode material 26 (which will be discussed in more detailed below) will not diffuse into the quartz. However, if silicon, gallium arsenide or indium phosphide is used, a passivation layer is desired.
  • a chromium layer 30 is deposited or flashed on the side portion 16 of the substrate 12.
  • an aluminum layer 28 is deposited over the Cr layer 30 and corresponds to the dimensions of the inner electrode 26.
  • Aluminum is an excellent isolation layer since temperature degradation tests indicate the AI-GaAs system to be very stable with an inner diffusion depth of only a few hundred angstroms at temperatures exceeding 500 degrees C.
  • the aluminum layer 28 and chromium layer 30 are only deposited under the region of inner electrode 26.
  • Inner electrode 26 may be made of gold or aluminum by way of example. Gold also diffuses rapidly into aluminum, and therefore an isolation layer may be required for this purpose.
  • a thin layer 32 of silicon dioxide which may be considered as part of passivation layer 24 can solve this isolation function.
  • the silicon dioxide isolation layer 32 is deposited over the entire edge of side portion 16 of substrate 12.
  • the silicon dioxide layer 32 may be used as an acoustic transformer, adjusting its thickness to help control the input impedance of the edge deposited transducer 10; in other words, the thickness of the isolation layer 32 may be varied for impedance matching purposes.
  • a starter layer 34 is deposited on passivation layer 24 for added adhesion and a nucleation site for the metallic inner electrode 26.
  • This layer 34 may be deposited by sputtering or evaporation.
  • This starter layer may be, by way of example, titanium (Ti), titanium tungsten (TiW) or chromium (Cr).
  • the inner metallic electrode 26 is deposited over starter layer 34 and may be deposited by sputtering or evaporation, by way of example. Various metals may be deposited, such as aluminum and gold, but gold is the preferred embodiment for the inner electrode 26 material. By depositing a gold film, a (111) orientation for the inner electrode 26 may be obtained. As can be seen more clearly in Figures 1 and 3, the gold metallic inner electrode 26 is only deposited under the outer metallic electrode 36 and not under bond pads 38. By depositing the inner electrode 26 only under the outer electrode 36, this prevents the formation of additional undesired acoustic transducers under bond pads 38. It also eliminates the potential capacitive shorting between inner electrode 26 and outer electrode 36 that would otherwise result. Both of these effects could greatly degrade the performance of transducer 10 since the area of bond pads 38 is much larger than the area of the outer electrode 36.
  • the next step in the fabrication of transducer 10 is the deposition of the transducer layer 40 over inner electrode 26.
  • Typical materials which can be used as the transducer layer 40 are zinc oxide (ZnO) and aluminum nitride (AIN). If gold is selected as the inner electrode material 26, then zinc oxide is preferred as the material for transducer layer 40 whereas if aluminum is chosen as the inner electrode 26, then aluminum nitride is preferred as the transducer material for transducer layer 40.
  • the zinc oxide layer may be sputtered directly onto the gold inner electrode 26, producing a highly C-axis-oriented zinc oxide film. With strong crystal alignment, the zinc oxide exhibits piezoelectric qualities and has a large piezoelectric coupling coefficient.
  • the last step in the fabrication process of the transducer 10 is the depositing of the outer metallic electrode 36 and bond pads 38. This may be accomplished by using direct-write e-beam lithography. To keep the response of transducer 10 as wide band as possible, it is desirable to keep the various layers of material as thin as possible. The thickness of transducer 10 can be reduced by using a material for outer electrode 36 which mechanically loads transducer 10. Because of this, and because of its high conductivity, gold is a preferred material. Bond pads 38 are kept as small as possible while still being large enough to bond to. A window 42 is formed in transducer layer 40 in order to expose a portion of inner electrode 26. An excitation source 44 is connected to the inner electrode 26 by conductor 46 while connected to outer electrode 36 by way of conductor 48 and bond pad 38.
  • the velocity profiles for the longitudinal and shear wave components of the surface wave in gallium arsenide is illustrated.
  • the magnitude of the shear component of the particle velocity is largest near the surface and the edge-bonded-transducer must be fabricated near the crystal surface to strongly couple to this component.
  • the edge-deposited transducer 10 constructed according to the present invention the largest coupling to the longitudinal component of the particle velocity in the gallium arsenide substrate occurs when transducer 10 is offset or is a predetermined distance from the top surface 18 of substrate 12.
  • the optimum position of outer electrode 36 of the transducer 10 is illustrated as the distance "a" from the top portion 18 of substrate 12.
  • This distance “a” is no higher than where the magnitude of the longitudinal component of the particle velocity in substrate 12 changes sign for the first time; this occurs at point 50 in Figure 4. If the top surface 52 ( Figure 4) of outer electrode 36 were to be positioned on the left side of point 50, performance of transducer 10 would be degraded due to the nulling out of a portion of the longitudinal component of the particle velocity of transducer 10, thereby reducing its efficiency.
  • Typical dimensions of an edge-deposited transducer 10 constructed according to the present invention using a zinc oxide transducer layer 40 and a gallium arsenide substrate 12, optimized for 40% bandwidth at a center frequency of 1.1 GHz is set forth in Table 1 below.

Landscapes

  • Physics & Mathematics (AREA)
  • Acoustics & Sound (AREA)
  • Surface Acoustic Wave Elements And Circuit Networks Thereof (AREA)
EP83304744A 1982-09-24 1983-08-16 Broad band surface acoustic wave edge deposited transducer Expired - Lifetime EP0104740B1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US422810 1982-09-24
US06/422,810 US4447754A (en) 1982-09-24 1982-09-24 Broad band surface acoustic wave edge deposited transducer

Publications (3)

Publication Number Publication Date
EP0104740A2 EP0104740A2 (en) 1984-04-04
EP0104740A3 EP0104740A3 (en) 1986-02-19
EP0104740B1 true EP0104740B1 (en) 1990-01-03

Family

ID=23676486

Family Applications (1)

Application Number Title Priority Date Filing Date
EP83304744A Expired - Lifetime EP0104740B1 (en) 1982-09-24 1983-08-16 Broad band surface acoustic wave edge deposited transducer

Country Status (4)

Country Link
US (1) US4447754A (forum.php)
EP (1) EP0104740B1 (forum.php)
JP (1) JPS5979621A (forum.php)
DE (1) DE3381071D1 (forum.php)

Families Citing this family (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH0388406A (ja) * 1989-04-11 1991-04-12 Sanyo Electric Co Ltd 弾性表面波素子
US5173667A (en) * 1991-02-19 1992-12-22 Ford Motor Company Acoustic wave transmission media delay line having internally disposed absorber channels
US5339101A (en) * 1991-12-30 1994-08-16 Xerox Corporation Acoustic ink printhead
US5365770A (en) * 1993-04-05 1994-11-22 Ford Motor Company Ultrasonic wave interferometers
ATE294373T1 (de) 1997-09-11 2005-05-15 Honeywell Inc Feststoff-flüssigkeits-interdiffusionsverbindun für ringlaserkreisel
EP1124328A1 (en) * 2000-02-10 2001-08-16 Lucent Technologies Inc. A method of fabricating a zinc oxide based resonator

Family Cites Families (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3935564A (en) * 1974-12-02 1976-01-27 The Board Of Trustees Of Leland Stanford, Jr. University Charge storage and monitoring apparatus utilizing acoustic waves
US4019200A (en) * 1975-06-11 1977-04-19 Rockwell International Corporation Monolithic surface acoustic wave signal storage device
US4211948A (en) * 1978-11-08 1980-07-08 General Electric Company Front surface matched piezoelectric ultrasonic transducer array with wide field of view
US4288775A (en) * 1979-11-09 1981-09-08 Bennewitz Paul F Device and method of manufacturing a relative humidity sensor and temperature sensor

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
"Acoustic fields and waves in solids" by B.A.Auld, Publ. John Wiley & Sons, 1973 pages 47, 92, 93. *

Also Published As

Publication number Publication date
EP0104740A3 (en) 1986-02-19
US4447754A (en) 1984-05-08
DE3381071D1 (de) 1990-02-08
JPH0334685B2 (forum.php) 1991-05-23
JPS5979621A (ja) 1984-05-08
EP0104740A2 (en) 1984-04-04

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